Processes for reducing the concentration of oxygen in gas mixtures comprising, for example, oxygen, olefinic compounds and other carbon-comprising compounds such as hydrocarbons are already known from the prior art.
DE 101 55 470 A1 describes a method for the synthesis of propylene oxide by epoxidation of propene with recovery of unreacted propene, in which propene is recovered from at least a portion of an off-stream of the propylene oxide synthesis by (i) addition of nitrogen to the off-gas stream, (ii) compression and (iii) condensation of the resulting stream, (iv) subjecting the stream to gas permeation and (v) separation. During condensation, a gas stream comprising propene, nitrogen and oxygen is separated from a liquid stream and fed to gas permeation. Addition of nitrogen is conducted so as to obtain a stream resulting from retentate of the gas permeation which has a low content of oxygen. Thus, formation of an ignitable mixture is avoided.
EP 0 719 768 A1 describes a process for recovering an olefin and oxygen which are comprised in an off-gas stream obtained from catalytic reaction of the olefin with hydrogen peroxide. In this separation process, the off-gas stream is contacted with an absorption agent such as isopropanol. In order to avoid ignitiable gas mixture, an inert gas like methane has to be added.
In his diploma thesis, K. Dehmel, University of Hamburg, Department of Physics, October 1999, discloses that a gas mixture of argon, CF4 and carbon dioxide can be freed from oxygen by catalytical hydrogenation of oxygen using hydrogen gas in the presence of a catalyst, namely copper oxide on a magnesium silicate-support. This document does not disclose gas mixtures comprising olefins and oxygen, from which oxygen shall be removed.
U.S. Pat. No. 6,204,218 B1 discloses a catalyst and a process for purifying streams of materials. The catalyst used comprises in its active composition from 0.05 to 1.0% by weight of at least one metal or compound of a metal of the 10th group of the Periodic Table of the Elements and from 0.05 to 1.0% by weight of at least one metal or compound of the metal of the 11th group of the Periodic Table of the Elements, with the weight ratio of the metal of the 11th group to the metal of the 10th group being from 0.95 to 1.05 and, as support, an SiO2-containing catalyst support. This catalyst can be used in processes for removing alkynes, dienes and/or monounsaturated hydrocarbons or oxygen from streams of materials.
U.S. Pat. No. 6,069,288 discloses a process for selectively separating hydrogen, or both hydrogen and carbon monoxide from olefinic hydrocarbons. This separation is achieved by contacting the mixture with oxygen over a catalyst at conditions sufficient to oxidize the hydrogen to form water while suppressing reaction of the reactive, unsaturated hydrocarbons. The catalyst contains at least one metal or metal oxide from groups IB, IIB, IIIB, IVB, VB, VIB, VIIB, and VIII of the Periodic Table of the Elements. In order to oxidize hydrogen present in the mixture which is to be treated, oxygen is added to this mixture in an amount being less than the stoichiometric amount required to react with the hydrogen, and optionally any carbon monoxide. In a second stage, the remaining carbon monoxide is reacted with water in a water gas-shift reaction to give carbon dioxide and hydrogen, and any remaining hydrogen is reacted with the small portion of the active unsaturated hydrocarbon present.
U.S. Pat. No. 4,869,883 discloses an inert gas purifier for bulk nitrogen without the use of hydrogen or other reducing gases. A three stage process is disclosed using copper, copper oxide and a molecular sieve adsorbent for the sequential removal of oxygen, hydrogen, carbon monoxide, carbon dioxide and water from an inert gas feed. The oxygen being present in the bulk inert gas stream is reacted with the carbon monoxide and hydrogen present in the bulk inert gas stream in the presence of a reduced copper containing catalyst at a temperature from 150 to 250° C., to form carbon dioxide and water. Unreacted carbon monoxide and hydrogen from this step are reacted with the oxygen component of a copper oxide containing catalysts at a temperature from 150 to 250° C. to form carbon dioxide, water, and reduced copper. Water and carbon dioxide are removed by adsorption on an adsorbent, preferably a molecular sieve adsorbent.
WO 2008/023051 A1 discloses a process for the elimination of oxygen, nitrogen oxides, acetylenes, and/or dienes from hydrogen-rich olefin-containing gas-mixtures. The gas-mixture further comprises hydrogen, one or more olefins, not being dienes, and optionally further components, and is brought into contact with a catalyst in a reaction zone, wherein the catalyst comprises copper(I) sulfide.
DE 19 29 977 discloses a process for the purification of olefins, comprising contacting a mixture comprising olefins, for example ethylene and propylene, in the gas phase with a catalytical mass comprising copper oxide (CuO) which can be regenerated subsequently with a gas comprising oxygen. Hydrogen is removed from said gas mixture by reaction with copper(I) oxide to obtain copper in elemental form and water. By treating copper in elemental form with oxygen gas, copper(II) oxide is formed again.
DE 32 28 023 A1 discloses a process for obtaining hydrocarbons from a gas mixture comprising 5 to 15% by volume oxygen, wherein the hydrocarbon is adsorbed at a suitable adsorbant. Prior to adsorption, the amount of oxygen can be reduced by catalytical reduction. In this reduction, a copper catalyst is used comprising copper in an amount of less than 5% by weight.
U.S. Pat. No. 5,446,232 discloses a further process for removing oxygen from hydrocarbon gases. With this method it is possible to remove oxygen from hydrogen, hydrocarbon or halogenated hydrocarbon gas containing from about 0.01 to about 10 mol-% oxygen. The oxygen removal is accomplished by contacting the gas with a hopcalite catalyst. Oxygen is removed by reaction with the hydrocarbons present in the mixture to form water and carbon dioxide. The catalyst is a mixture of CuO and MnO2.
WO 2004/033598 A1 describes a process for the removal of oxygen from a gas mixture comprising oxygen, at least one olefin, hydrogen, carbon monoxide and optionally at least one alkyne wherein the ratio of oxygen:hydrogen in the gas mixture is 1 part by volume of oxygen to at least 5 parts of volume of hydrogen, i.e., the volume ratio of oxygen to hydrogen must be smaller than or equal to 0.2. Accordingly, examples 9 and 10 of WO 2004/033598 A1 disclose gas streams having a molar oxygen:hydrogen ratio of 0.0034, i.e. a molar hydrogen:oxygen ratio of 294, and examples 11 and 12 disclose gas streams having an oxygen:hydrogen ratio of 0.0052, i.e. a molar hydrogen:oxygen ratio of 192. The process comprises contacting the gas mixture with the catalyst in a reaction zone under conditions sufficient to oxidize at least a portion of the hydrogen and at least a portion of the carbon monoxide, without significant hydrogenation of the at least one olefin. The catalyst comprises at least one metal selected from the group consisting of the 10th group and the 11th group of the Periodic Table of the Elements, the metal or oxide of the metal being supported on an oxide support, provided that if the catalyst comprises at least one metal or oxide of metal from the 10th group supported on an oxide support, the catalyst also comprises tin and provided that if the catalyst comprises at least one metal or oxide of metal of the 11th group, the oxide support is a zeolite. The gas mixtures subjected to the process of WO 2004/033598 A1 are typically obtained from steam cracking of hydrocarbons, dehydrogenation of paraffinic feedstock, conversion of methanol to olefins, and auto-thermal cracking of hydrocarbons. The process of WO 2004/033598 A1 is particularly suitable for gas mixtures comprising from greater than 0 up to and including 60 percent by volume olefin. Advantageously, the process of WO 2004/033598 A1 enables oxygen to be removed from gas mixtures containing low levels of oxygen such as 2000 ppm or less, and especially from gas mixtures having a low concentration of oxygen and a high concentration of hydrogen such as at least 10 percent by volume of hydrogen or for example greater than 40 percent by volume of hydrogen.
In WO 01/51475 A1, a process for working up a mixture comprising an alkene and oxygen is disclosed, wherein oxygen is removed from the mixture by nondestillative methods. According to this document, oxygen can be removed by combustion or by reaction of the oxygen with at least one suitable chemical compound or by a combination of these methods. The combustion of oxygen present in the mixture is conducted in the presence of a copper-chromite catalyst. The at least one suitable compound with which oxygen is reacted is an alkane. The oxydehydrogenation reaction of an alkane and oxygen gives rise to the corresponding alkene. WO 01/51475 A1 does not disclose a process for removing oxygen from a mixture comprising propene in which hydrogen is added and oxygen and hydrogen react to give water in the presence of a catalyst.
Accordingly, the prior art describes, on the one hand, industrial processes such as dehydrogenation processes in which gas mixtures are obtained containing oxygen, hydrogen, olefin and optionally alkanes in mutual ratios which are fundamentally different from the gas mixtures obtained from epoxidation reactions such as epoxidation of propene. On the other hand, the prior art describes catalysts which do not meet the specific requirements of removing oxygen from gas mixtures obtained in epoxidation reactions such as epoxidation of propene.
Moreover, adsorption techniques described in the prior art have the major disadvantage that during adsorption of propene, the explosive range of propene/oxygen mixtures is passed due to the increasing concentration of oxygen. Consequently, in order to avoid process risks, apparatuses used for sorption techniques have to be constructed highly pressure resistant, thus causing high costs which in turn render the overall process economically undesirable. Alternatively or additionally, at least one suitable inert gas would have to be added, with additional costs for such further chemical compounds incurred. Furthermore, known methods based on reactive adsorption, especially as far as adsorption of oxygen is concerned, require the periodical regeneration of the adsorbent.
Therefore, it is an object of the present invention to provide a process for producing propylene oxide in which an effective removal of oxygen from gas mixtures directly or indirectly obtained from the epoxidation reaction of propene is achieved.
It is another object of the present invention to provide a catalyst for use in a work-up stage of a process for producing propylene oxide, in which work-up stage oxygen is effectively removed from a gas mixture.
It is still another object of the present invention to provide a work-up stage in a process for producing propylene oxide, in which work-up stage oxygen is effectively removed from a gas mixture comprising oxygen and propene wherein the disadvantages of sorption process are avoided.
It is still another object of the present invention to provide a work-up stage in a process for producing propylene oxide, in which work-up stage oxygen is effectively removed from a gas mixture comprising oxygen and propene by a specifically adapted catalyst in combination with a specifically adapted addition of hydrogen and reaction conditions which minimizes losses of propene due to hydrogenation.